1. Field of the Invention
The present invention pertains generally to methods and systems for manufacture and characterization of optical components. More specifically, the invention is a method for real time process control, using a linearly swept tunable laser, which allows fast, easy in-situ monitoring and control, as well as post-processing characterization, of wavelength-specific properties of optical components.
2. Description of the Background Art
In telecommunications, specifically with wavelength division multiplexing (WDM), the wavelength characterization of components is critical for proper performance. As more channels are added to WDM systems with narrower channel spacings, the accuracy of wavelength characterization must improve.
The wavelength specific properties of optical components are typically characterized by post-manufacturing testing. One form of such testing involves a step-and-measure approach using tunable lasers. In this technique, a control system sequentially directs a tunable laser to selected wavelengths in discrete steps, with optical characterization carried out at each such step. Even with an effective control system and reliable tunable laser source, each such step generally has a duration on the order of 100 to 500 milliseconds. For a typical optical component which must be wavelength-characterized at sub-nanometer intervals over a range of several nanometers, the testing time required for each individual component thus be several minutes or tens of minutes in duration. For example, where a component is scanned over a hundred nanometer range at ten picometer increments, 10,000 steps are involved and a duration of 5000 seconds, or more than 83 minutes, is required.
Wavelength specific testing of optical components has also been carried out through use of an optical spectrum analyzer (OSA) with an incoherent white light source. The use of an optical spectrum analyzer is very slow, however, and is limited in resolution. Use of optical spectrum analyzer techniques is adequate for filters greater than 0.8 nm (100 GHz) in width, but is not effective for the narrower filter responses necessary for current technologies. The slow characterization time and poor resolution of currently available wavelength characterization techniques greatly increases the time and expense associated with manufacture of wavelength-specific optical components, and have limited optical characterization to post manufacturing quality control environments.
There is accordingly a need for a method for wavelength specific characterization of optical components which is fast, which provides high wavelength resolution, and which can be used in real-time process control during manufacture of optical components. The present invention satisfies these needs, as well as others, and generally overcomes the deficiencies found in the background art.
An object of the invention is to provide a real-time process control system for optical components which allows high speed characterization of wavelength-specific component properties.
Another object of the invention is to provide a real-time process control system for optical components which has high wavelength resolution.
Another object of the invention is to provide a real-time process control system for optical components which allows in situ monitoring and control of wavelength specific component properties during manufacture.
Another object of the invention is to provide a real-time process control system for optical components which utilizes a fast, linear swept tunable laser.
Another object of the invention is to provide a real-time process control system for optical components which delivers high power and an effective dynamic range.
Another object of the invention is to provide a real-time process control system for optical components usable for narrow filter responses.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing the preferred embodiment of the invention without placing limitations thereon.
The present invention is a system and method for real time process control, using a linearly swept tunable laser, which allows high speed in-situ monitoring and control of wavelength- or frequency-specific properties of optical components. In its most general terms, the invention comprises scanning an optical component with a high speed, high linearity tunable laser, and detecting optical output from the component during the scanning. Preferably, the invention also includes adjusting or controlling the optical properties of the component during scanning, according to detected optical output from the component.
By way of example, and not necessarily of limitation, the invention is embodied in a process control system comprising a high speed, high linearity, tunable laser which is operatively coupled to the optical component under characterization. The optical component is operatively coupled to an optical detector. A system control processor is operatively coupled to the tunable laser and detector. Preferably, a manufacturing or processing control unit is associated with the optical component and is operatively coupled to the system control processor.
In operation, the system control processor initiates a fast, linear scan of the optical component by the tunable laser, over a selected wavelength or frequency range and at a selected wavelength tuning rate. During scanning, output from the component is detected by the detector, converted to a digital signal, and provided to the system control processor. Scanning and detection may be carried out via transmission or reflection, according to the particular use of the invention. The system control processor may, responsive to feedback from the detector, direct the manufacturing control unit to physically control a manufacturing operation or operations associated with the optical component, according to the feedback from the detector. The system control processor may additionally, in response to feedback from the detector, adjust the scan speed or range of the tunable laser.
A high scan speed or rate for the tunable laser is important to operation of the invention. Preferably, the laser is tuned at a rate of at least 100 nanometers per second during scanning of the optical component, and may be tuned at a scanning rate within the range of between 5 picometers per second and 40,000 nanometers per second. The linearity and repeatability of laser tuning during scanning is also important. Preferably, tuning linearity is such that a wavelength error of no greater than 0.1 nanometer from best linear fit is provided during scanning, and more preferably of no greater than 0.01 nanometer from best linear fit.
One preferred tunable laser for use with the invention, which provides suitable tuning speed and tuning linearity, is a cam-driven external cavity diode laser (ECDL) apparatus wherein a movable prism is positionally adjusted with respect to a grating according to the action of a cam body on a tuning arm coupled to the prism. The structure and configuration of the cam is controlled to provide high speed, high linearity tuning output from the ECDL apparatus. Various other tunable laser devices may also be used with the invention.
The high tuning speed and high degree of scan linearity as provided by the invention allows high speed real time, in-situ characterization and control of wavelength specific properties of optical components during manufacture of the components. The real time processing control of components during manufacturing according to the invention provides shorter manufacturing times and lower manufacturing costs for optical components than has been achievable heretofore.